Description of the Prior Art
Differential comparators are well known in the integrated circuit (IC) art for providing an output signal that depends on the relative levels of the inputs signals. For example, referring to comparator 10 in FIG. 1, when the voltage on the first (non-inverting) input 11 is higher than the voltage on the second (inverting) input 12, the comparator output 13 is high. Alternatively, when the voltage on the first input is less than the voltage on the second input, the comparator output is low.
The difference between the voltages on the first and second inputs is referred to as the "differential input voltage" (AV). One figure of merit of a differential comparator is how large the differential input voltage must be in order to cause the output voltage to change from high to low, or vice-versa. Another figure of merit is referred to as the "Common Mode Range" (CMR), which is the voltage range over which a small differential input signal can be detected. Most differential comparators have a rather limited CMR as compared to the full power supply voltage range, often referred to as the "rail-to-rail" voltage range. Typically, a comparator with p-channel field effect transistor input devices has a CMR from the negative power supply voltages V.sub.SS (0 volts) to about V.sub.DD -1.5 volts. A comparator with n-channel field effect transistor input devices has a CMR from about 1.5 volts to V.sub.DD.
However, several applications exist where a relatively larger CMR is necessary. For example, the Universal Serial Bus (USB) specification requires a receiver with a rail-to-rail Common Mode Range. The Low Voltage Differential Signal (LVDS) specification also requires a receiver with a very wide range, too wide for typical prior-art comparators to meet. One design that achieves a relatively large CMR range is given in the IEEE Journal of Solid-State Circuits, Vol 30, February 1995, pp. 156-159. However, that design is relatively complicated and requires a large amount of integrated circuit (IC) chip area, being more similar in many respects to an operational amplifier than to a comparator.